Conventionally, there are demands for a reduction in power consumption in a display device such as a liquid crystal display device. In recent years, hence, developments have been made in a driving method involving “providing a pausing period between a writing period and a writing period in order to pause a write operation by bringing all gate bus lines (scanning signal lines) into a non-scanning state” for a liquid crystal display device. It should be noted that the writing period refers to a period for charging a pixel capacitance in a display unit, based on an image signal in one frame (one screen). The writing period is also called, for example, a scanning period, a charging period, or a refreshing period. According to the driving method described above, there is no need to apply, for example, a controlling signal to a liquid crystal drive circuit (e.g., a gate driver, a source driver) in the pausing period. Therefore, a drive frequency of the liquid crystal drive circuit is reduced as a whole, so that a reduction in power consumption can be realized. It should be noted that the driving method involving providing the pausing period for pausing the write operation is called, for example, “low-frequency driving” or “pause driving”. FIG. 14 is a diagram for illustrating one example of the low-frequency driving. In a liquid crystal display device that employs the low-frequency driving, as shown in FIG. 14, for example, a writing period having a length corresponding to one frame period (one frame period: 16.67 ms) in a general liquid crystal display device having a refresh rate (a drive frequency) of 60 Hz and a pausing period having a length corresponding to a 59-frame period appear alternately. This low-frequency driving is suitable for still image display.
In recent years, attention has been given to a thin-film transistor using an oxide semiconductor as a channel layer (hereinafter, such a thin-film transistor is referred to as an “oxide TFT”). The oxide TFT has an off-leak current (i.e., a current to be flown in an OFF state) which is considerably smaller than that of a thin-film transistor using, for example, amorphous silicon as a channel layer (hereinafter, such a thin-film transistor is referred to as a “silicon-based TFT”). Therefore, a liquid crystal display device using an oxide TFT as an element in a liquid crystal panel is capable of holding a voltage written on a pixel capacitance, for a relatively long period of time. Accordingly, the low-frequency driving described above is particularly employed for a liquid crystal display device using the oxide TFT as an element in a liquid crystal panel. The low-frequency driving is occasionally employed for a liquid crystal display device using the silicon-based TFT as an element in a liquid crystal panel.
A liquid crystal has a characteristic in that the liquid crystal is degraded when being successively applied with a direct-current voltage. Accordingly, in order to suppress the degradation of a liquid crystal, a liquid crystal display device performs alternating-current driving of reversing the polarity of a pixel voltage (liquid crystal applied voltage). A driving scheme called frame-reversal driving of reversing the polarities of pixel voltages every frame in a state in which the polarities of the pixel voltages are made equal to one another in all pixels is known as an alternating-current driving scheme. It should be noted that, hereinafter, the driving scheme of reversing the polarity of a pixel voltage every predetermined period is referred to as a “reversal driving scheme”. However, the frame-reversal driving relatively tends to cause flicker upon display of an image. So, conventionally, reversal driving schemes with various polarity reversal patterns have been employed in order to suppress the occurrence of flicker. Typically, line-reversal driving, column-reversal driving, and dot-reversal driving are known as the reversal driving scheme.
The line-reversal driving refers to a driving scheme of reversing the polarity of a pixel voltage every frame and every predetermined number of gate bus lines. When the polarity of the pixel voltage is reversed every frame and every gate bus line in this driving scheme, the polarities of pixel voltages on 4-by-4 pixels in a certain frame are as shown in FIG. 15. The polarities of the pixel voltages on all the pixels are reversed in a subsequent frame.
The column-reversal driving refers to a driving scheme of reversing the polarity of a pixel voltage every frame and every predetermined number of source bus lines. When the polarity of the pixel voltage is reversed every frame and every source bus line in this driving scheme, the polarities of pixel voltages on 4-by-4 pixels in a certain frame are as shown in FIG. 16. It should be noted that the polarities of the pixel voltages on all the pixels are reversed in a subsequent frame.
The dot-reversal driving refers to a driving scheme of reversing the polarity of a pixel voltage every frame and also reversing the polarities of adjacent pixels in vertical and horizontal directions. With regard to this driving scheme, the polarities of pixel voltages on 4-by-4 pixels in a certain frame are as shown in FIG. 17. It should be noted that the polarities of the pixel voltages on all the pixels are reversed in a subsequent frame. According to the dot-reversal driving, a polarity reversal pattern becomes complicated as compared with the line-reversal driving and the column-reversal driving. Therefore, the occurrence of flicker is effectively suppressed.
It should be noted that, in relation to this invention, Japanese Patent Application Laid-Open No. 2006-126475 discloses the invention of a liquid crystal display device capable of reducing power consumption and suppressing heat generation while suppressing the occurrence of flicker. In this liquid crystal display device, a reversal driving scheme is decided based on the gradation of input image data every plurality of pixels of a frame less than one frame.